Referring to FIG. 1, a conventional locking lever LL for actuating a door lock device of an automobile has a torsion spring TS coupled thereto. When the lever LL is operated to lock or unlock the device, the force necessary for the operation assumes a maximum value immediately before a dead point is reached during its stroke, by the action of the spring TS.
Referring next to FIG. 2, this force, or load, is indicated by solid line A which surrounds a hatched region. When the lever is actuated to lock the device, the load which is taken on the ordinate of this graph goes in the positive direction with increasing stroke which is taken on the abscissa. When the lever is actuated to unlock the device, the load goes in the negative direction with increasing stroke. It can be understood from this graph that a force is needed until the dead point is reached in whichever direction the lever is moved, and that a large force is necessary at the beginning of the stroke. The required force then assumes a maximum value with a slight additional stroke.
In an ordinary solenoid device having a single field coil, a plunger attracted by the coil, and a returning spring, the attracting force increases as tne plunger is attracted, as indicated by phantom line B in FIG. 2. Hence, in order to set the force needed at the beginning of the stroke greater than a required force such as the peak value of the curve A, the solenoid device is necessarily made quite large. In the type of device where the plunger is repelled by a field coil, the reverse situation takes place. However, in order to obtain a force greater than the maximum value at a given stroke, a large initial driving force is required to be produced, as indicated by phantom line C in FIG. 2. Therefore, this kind of solenoid device is also made bulky.
In view of the foregoing considerations, solenoid devices producing a driving force whose characteristic curve is similar to the curve A have been proposed. One kind of such conventional devices is shown in FIG. 3, in which a shaft 4 extends through a disk-like permanent magnet 1 of ferrite and through magnetic cores 2 and 3, which are shaped into the form of a truncated cone and are disposed on opposite sides of the magnet 1. This magnet 1 is magnetized with its north and south poles at its two ends. The shaft is provided with annular grooves with which E-rings 5 and 6 engage. These rings support the cores 2 and 3, respectively. Disposed outside of these rings are rubber disks 7 and 8 to absorb mechanical impact. The shaft 4 also extends through these disks 7 and 8. Field coils 9 and 10 are wound on bobbins 11 and 12, respectively. The bobbin 11 is supported by one end plate 13 and the center plate 15 of a magnetic yoke. Similarly, the bobbin 12 is supported by the other end plate 14 and the center plate 15 of the yoke. These bobbins 11 and 12 are housed in the body 16 of the yoke in the form of a cylindrical casing. Both ends of the casing 16 is crimped inwardly such that the end plates 13, 14, the bobbins 11, 12, center plate 15, and the casing 16 are joined together.
When an electric current is supplied in the direction indicated by the arrow A in FIG. 3, the end plate 13 and 14 of the yoke are magnetized to exhibit north poles, while the center plate 15 is magnetized to exhibit a south pole. Since the left and right sides of the permanent magnet 1 are south and north poles, respectively. Accordingly, when the device is energized with the current flowing in the direction indicated by the arrow A, the plunger core 5 is attracted towards the end plate 13 and, at the same time, it is repelled by the center plate 15, whereby the core 5 is urged in the direction indicated by the arrow B. Likewise, the plunger core 3 is repelled by the end plate 14 while attracted by the center plate 15, so that the core 3 is also urged in the same direction. Thus, these cores push the shaft 4 to move it in the direction indicated by the arrow B until the rubber disk 7 abuts on the end plate 13, at which time the shaft comes to a halt. After this movement of the shaft 4 to the left (in the direction indicated by the arrow B), the current supplied in the direction indicated by the arrow A is reversed. Then, the end plates 13 and 14 are polarized south, while the center plate 15 is polarized north. This moves the shaft 4 to the right (in the opposite direction to the direction B), and then it halts in the condition shown in FIG. 3. The solenoid device thus far described is used as a driving source for automatically locking and unlocking a vehicle door, for instance.
In this kind of solenoid device where the plunger is disposed in the space inside of the coils and is driven by tne attracting and repelling forces of the magnetic field set up by the coils, the fringes of the cylindrical casing, or the main yoke, are crimped so as to be firmly secured to the end plates 13 and 14, as shown in FIG. 3, such that the end plates 13, 14, the bobbins 11, 12, the center plate 15, and the main yoke 16 are joined together. In this structure, the gap between the end plates 13 and 14, more specifically the gap between the end plate 13 and the center plate 15 and the gap between the end plate 14 and the center plate 15, is determined by the dimensions of the end plates 13, 14, the bobbins 11, 12, the main yoke 16, the center plate 15, the strength of the crimping at both ends of the yoke 16, and the direction of the applied pressure. In this way, the parameters which affect the gap are numerous, and therefore the error varies greatly from product to product. Especially, the error of products attributable to the crimping poses a serious problem. Further, since the crimping applies a force to the coil bobbins, these bobbins are forced to have a large wall thickness. This introduces such an undesirable situation that the diameter of the solenoid is large.